Plasma reactor apparatus with a VHF capacitively coupled plasma source of variable frequency

ABSTRACT

A plasma reactor for processing a workpiece includes a reactor chamber and a workpiece support within the chamber, the chamber having a ceiling facing the workpiece support, a capacitively coupled plasma source power applicator comprising a source power electrode at one of: (a) the ceiling (b) the workpiece support, and plural VHF power generators of different fixed frequencies coupled to the capacitively coupled source power applicator, and a controller for independently controlling the power output levels of the plural VHF generators so as to control an effective VHF frequency applied to the source power electrode. In a preferred embodiment, the reactor further includes a plasma bias power applicator that includes a bias power electrode in the workpiece support and one or more RF bias power generators of different frequencies coupled to the plasma bias power applicator.

BACKGROUND OF THE INVENTION

In semiconductor fabrication processes, conventional sources of plasmasource power, such as inductively coupled RF power applicators orcapacitively couple RF power applicators, introduce inherent plasmadensity non-uniformities into the processing. In particular, inductivelycoupled plasma sources are characterized by an “M”-shaped radialdistribution of plasma ion density over the semiconductor workpiece orwafer. As device geometries have continued to shrink, suchnon-uniformities become more critical, requiring better compensation.Presently, the non-uniformity of an overhead inductively coupled sourceis reduced or eliminated at the wafer surface by optimizing the coildesign and ceiling-to-wafer distance, aspect ratio, of the chamber. Thisdistance must be sufficient so that diffusion effects can overcome theeffects of the nonuniform ion distribution in the ion generation regionbefore they reach the wafer. For smaller device geometries on the waferand the inductive plasma source located near the ceiling, a largeceiling-to-wafer distance is advantageous. However, a largeceiling-to-wafer distance can prevent the beneficial gas distributioneffects of a ceiling gas distribution showerhead from reaching the wafersurface, due to diffusion over the large distance. For such largeceiling-to-wafer distances, it has been found that the gas distributionuniformity is not different whether a gas distribution showerhead isemployed or a small number of discrete injection nozzles are employed.

In summary, the wafer-ceiling gap is optimized for ion densityuniformity which may not necessarily lead to gas delivery optimization.

One limitation of such reactors is that not all process parameters canbe independently controlled. For example, in an inductively coupledreactor, in order to increase reaction (etch) rate, the plasma sourcepower must be increased to increase ion density. But, this increases thedissociation in the plasma, which can reduce etch selectivity andincrease etch microloading problems, in some cases. Thus, the etch ratemust be limited to those cases where etch selectivity or microloadingare critical.

Another problem arises in the processing (e.g., etching) of multi-layerstructures having different layers of different materials. Each of theselayers is best processed (e.g., etched) under different plasmaconditions. For example, some of the sub-layers may be best etched in aninductively coupled plasma with high ion density and high dissociation(for low mass highly reactive species in the plasma). Other layers maybe best etched in a capacitively coupled plasma (low dissociation, highmass ions and radicals), while yet others may be best etched in plasmaconditions which may be between the two extremes of purely inductivelyor capacitively coupled sources. However, to idealize the processingconditions for each sub-layer of the structure being etched wouldrequire different process reactors, and this is not practical.

SUMMARY OF THE INVENTION

A plasma reactor for processing a workpiece includes a reactor chamberand a workpiece support within the chamber, the chamber having a ceilingfacing the workpiece support, a capacitively coupled plasma source powerapplicator comprising a source power electrode at one of: (a) theceiling (b) the workpiece support, and plural VHF power generators ofdifferent fixed frequencies coupled to the capacitively coupled sourcepower applicator, and a controller for independently controlling thepower output levels of the plural VHF generators so as to control aneffective VHF frequency applied to the source power electrode. In apreferred embodiment, the reactor further includes a plasma bias powerapplicator that includes a bias power electrode in the workpiece supportand one or more RF bias power generators of different frequenciescoupled to the plasma bias power applicator.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a simplified block diagram of a plasma reactor in accordancewith an embodiment of the invention.

FIGS. 2A and 2B together constitute a block diagram depicting a methodof one embodiment of the invention, and these drawings are hereinafterreferred to collectively as “FIG. 2”.

FIG. 3A is a graph depicting a radial distribution of plasma ion densitythat is typical of an inductively coupled plasma.

FIG. 3B is a graph depicting the radial distribution of plasma iondensity that is typical of a capacitively coupled plasma.

FIG. 3C is a graph depicting the radial distribution of plasma iondensity obtained in the reactor of FIG. 1 in accordance with a method ofthe invention.

FIG. 4 illustrates ion radial distribution non-uniformity (deviation) asa function of the ratio of the power levels of inductively andcapacitively coupled power.

FIG. 5 illustrates ion radial distribution non-uniformity (deviation) asa function of the ratio of the pulse duty cycles of inductively andcapacitively coupled power.

FIG. 6 is a graph illustrating lines of constant plasma ion density forpairs of values of inductively and capacitively coupled power levels.

FIG. 7 is a graph illustrating lines of constant plasma ion density forpairs of values of inductively and capacitively coupled power pulsedduty cycles.

FIG. 8 is a graph illustrating the dependency of electron density in thebulk plasma as a function of source power levels for different VHFfrequencies of the capacitively coupled power.

FIGS. 9A and 9B together constitute a block diagram depicting a methodof another embodiment of the invention, and are hereinafter referred tocollectively as “FIG. 9”.

FIG. 10 is a graph illustrating different bulk plasma electron energydistribution functions obtained for different mixtures of capacitivelyand inductively coupled power.

FIG. 11 depicts the change in electron energy distribution functions fordifferent source power levels obtained when capacitively coupled poweris added to inductively coupled power.

FIG. 12 depicts different optical emission spectra obtained fordifferent degrees of dissociation (electron energy distributions).

FIG. 13 is a graph depicting how the degree of dissociation (e.g.,population of free carbon or free fluorine) increases with increasingratio of inductively coupled power to capacitively coupled power.

FIG. 14 is a graph depicting how the degree of dissociation (e.g.,population of free carbon or free fluorine) increases with increasingratio of inductively coupled power pulsed duty cycle to capacitivelycoupled power duty cycle.

FIGS. 15A and 15B illustrate the contemporaneous waveforms of pulsedinductively coupled power and capacitively coupled power, respectively.

FIG. 16 is a graph illustrating how the degree of dissociation decreaseswith increasing frequency of capacitively coupled power.

FIGS. 17A, 17B and 17C are graphs of sheath ion energy distribution forthe cases in which only low frequency bias power is applied, only highfrequency bias power is applied and both low and high frequency biaspower is applied to the wafer, respectively.

FIG. 18 illustrates a multi-layer gate structure which is to be etchedin the process of FIG. 2 or FIG. 9.

FIG. 19 illustrates a plasma reactor in accordance with a firstembodiment.

FIGS. 20 and 21 illustrate different implementations of a ceilingelectrode in the reactor of FIG. 19.

FIGS. 22 and 23 illustrate different embodiments of the inductiveantenna of the reactor of FIG. 19.

FIG. 24 illustrates a plasma reactor in accordance with anotherembodiment.

FIG. 25 illustrates a plasma reactor in accordance with yet anotherembodiment.

FIG. 26 illustrates a plasma reactor in accordance with a furtherembodiment.

FIG. 27 illustrates a plasma reactor in accordance with a yet furtherembodiment.

FIG. 28 illustrates a plasma reactor in accordance with anotherembodiment.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 depicts a plasma reactor for processing a workpiece 102, whichmay be a semiconductor wafer, held on a workpiece support 103, which may(optionally) be raised and lowered by a lift servo 105. The reactorconsists of a chamber 104 bounded by a chamber sidewall 106 and aceiling 108. The ceiling 108 may comprise a gas distribution showerhead109 having small gas injection orifices 110 in its interior surface, theshowerhead 109 receiving process gas from a process gas supply 112. Inaddition, process gas may be introduced through gas injection nozzles113. The reactor includes both an inductively coupled RF plasma sourcepower applicator 114 and a capacitively coupled RF plasma source powerapplicator 116. The inductively coupled RF plasma source powerapplicator 114 may be an inductive antenna or coil overlying the ceiling108. In order to permit inductive coupling into the chamber 104, the gasdistribution showerhead 109 may be formed of a dielectric material suchas a ceramic. The VHF capacitively coupled source power applicator 116is an electrode which may be located within the ceiling 108 or withinthe workpiece support 103. In an alternative embodiment, thecapacitively coupled source power applicator 116 may consist of anelectrode within the ceiling 108 and an electrode within the workpiecesupport 103, so that RF source power may be capacitively coupled fromboth the ceiling 108 and the workpiece support 103. (If the electrode iswithin the ceiling 108, then it may have multiple slots to permitinductive coupling into the chamber 104 from an overhead coil antenna.)An RF power generator 118 provides high frequency (HF) power (e.g.,within a range of about 10 MHz through 27 MHz) through an optionalimpedance match element 120 to the inductively coupled source powerapplicator 114. Another RF power generator 122 provides very highfrequency (VHF) power (e.g., within a range of about 27 MHz through 200MHz) through an optional impedance match element 124 to the capacitivelycoupled power applicator 116. The efficiency of the capacitively coupledpower source applicator 116 in generating plasma ions increases as theVHF frequency increases, and the frequency range preferably lies in theVHF region for appreciable capacitive coupling to occur. As indicatedsymbolically in FIG. 1, power from both RF power applicators 114, 116 iscoupled to a bulk plasma 126 within the chamber 104 formed over theworkpiece support 103. RF plasma bias power is capacitively coupled tothe workpiece 102 from an RF bias power supply coupled to (for example)an electrode 130 inside the workpiece support and underlying the wafer102. The RF bias power supply may include a low frequency (LF) RF powergenerator 132 and another RF power generator 134 that may be either amedium frequency (MF) or a high frequency (HF) RF power generator. Animpedance match element 136 is coupled between the bias power generators132, 134 and the workpiece support electrode 130. A vacuum pump 160evacuates process gas from the chamber 104 through a valve 162 which canbe used to regulate the evacuation rate. The evacuation rate through thevalve 162 and the incoming gas flow rate through the gas distributionshowerhead 109 determine the chamber pressure and the process gasresidency time in the chamber.

The plasma ion density increases as the power applied by either theinductively coupled power applicator 114 or VHF capacitively coupledpower applicator 116 is increased. However, they behave differently inthat the inductively coupled power promotes more dissociation of ionsand radicals in the bulk plasma and a center-low radial ion densitydistribution. In contrast, the VHF capacitively coupled power promotesless dissociation and a center high radial ion distribution, andfurthermore provides greater ion density as its VHF frequency isincreased.

The inductively and capacitively coupled power applicators may be usedin combination or separately, depending upon process requirements.Generally, when used in combination, the inductively coupled RF powerapplicator 114 and the capacitively coupled VHF power applicator 116couple power to the plasma simultaneously, while the LF and HF biaspower generators simultaneously provide bias power to the wafer supportelectrode 130. As will be discussed below, the simultaneous operation ofthese sources enables independent adjustment of the most importantplasma processing parameters, such as plasma ion density, plasma ionradial distribution (uniformity), dissociation or chemical speciescontent of the plasma, sheath ion energy and ion energy distribution(width). For this purpose, a source power controller 140 regulates thesource power generators 118, 122 independently of one another (e.g., tocontrol their ratio of powers) in order to control bulk plasma iondensity, radial distribution of plasma ion density and dissociation ofradicals and ions in the plasma, as will be described in a later portionof this specification. The controller 140 is capable of independentlycontrolling the output power level of each RF generator 118, 122. Inaddition, or alternatively, the controller 140 is capable of pulsing theRF output of either one or both of the RF generators 118, 122 and ofindependently controlling the duty cycle of each, or of controlling thefrequency of the VHF generator 122 and, optionally, of the HF generator118. In addition, a bias power controller 142 controls the output powerlevel of each of the bias power generators 132, 134 independently inorder to control both the ion energy level and the width of the ionenergy distribution, as will be described below. The controllers 140,142 are operated to carry out various methods of the invention.

In accordance with a first method of the invention depicted in FIG. 2,plasma ion density, plasma ion density uniformity, sheath ion energy andion energy distribution (width) are controlled independently of oneanother. The method of FIG. 2 includes introducing process gas,preferably through the ceiling gas distribution showerhead 109 (block202 of FIG. 2). The method continues by capacitively coupling VHF sourcepower to the bulk plasma (block 204) while inductively coupling RFsource power to the bulk plasma (block 206). The user establishes acertain plasma ion density in accordance with a particular process step.This is accomplished by maintaining the combined total of the VHFcapacitively coupled source power and the inductively coupled sourcepower at a level providing the desired plasma ion density for theprocess step to be carried out (block 208). At the same time, the radialdistribution of plasma ion density at the wafer surface is customized(e.g., to make as uniform as possible) while maintaining the desiredplasma ion density. This is accomplished by adjusting the ratio betweenthe amounts of the VHF capacitively coupled power and the inductivelycoupled power (block 210). This apportions the radial ion distributionbetween the center-low distribution promoted by the inductively coupledpower and the center-high distribution promoted by the VHF capacitivelycoupled power. As will be described below in this specification, thiscan be accomplished without perturbing the ion density by maintainingthe total RF power nearly constant while changing only the ratio betweenthe power delivered by the HF and VHF generators 118, 122.

The adjustment of step 210 can be carried out by any one (or acombination) of the following steps: A first type of adjustment consistsof adjusting the RF generator power levels of the inductively andcapacitively coupled power sources 118, 122 (block 210 a of FIG. 2).Another type consists of pulsing at least one or both of the inductivelyand capacitively coupled RF power generators 118, 122 and adjusting theduty cycle of one relative to the other (block 210 b of FIG. 2). A thirdtype consists of adjusting the effective frequency of the capacitivelycoupled power VHF generator 122 (block 210 c of FIG. 2), in which plasmaion density increases as the VHF frequency is increased. Adjusting theeffective VHF frequency of the capacitively coupled plasma source powermay be accomplished in a preferred embodiment by providing two VHFgenerators 122 a, 122 b of fixed but different VHF frequencies (i.e., anupper VHF frequency f₁ output by the generator 122 a and a lower VHFfrequency f₂ output by the generator 122 b) whose combined outputs areapplied (through impedance matches 124 a, 124 b) to the capacitive powerapplicator. Changing the effective VHF frequency f_(eff) within a rangebounded by the upper and lower frequencies f₁, f₂, is performed byvarying the ratio between the output

power levels a₁, a₂, of the two generators 122 a, 122 b. The effectivefrequency f_(eff) may be approximated to first order as a function ofthe frequencies f₁ and f₂ of the two VHF generators 122 a, 122 b,respectively, and their respective adjustable output power levels, a₁and a₂, as follows: f_(eff)=(a₁f₁+f₂a₂)/(a₁+a₂). While the foregoingexample involves two VHF generators, a larger number may be employed ifdesired.

The VHF capacitive source can efficiently create plasma density withoutcreating high RF voltages in the plasma, which is similar to aninductively coupled plasma (ICP) source. In contrast, the LF and HF biassources efficiently create high RF voltages in the plasma but contributelittle to plasma density. Therefore, the combination of the VHF source(or VHF sources) and the ICP source allows the plasma to be producedwithout the side effect of creating large RF voltages within the plasma.As a result, the RF voltage produced by the LF of HF source applied towafer pedestal can operate independently from the plasma densitycreating source. The VHF source can be operated independently from theICP source, with an ability to create plasma density in combination withthe ICP (whereas the traditional ICP source employs an HF or LFcapacitively coupled power source connected to the wafer pedestal tocreate RF voltage on the wafer only).

The method further includes coupling independently adjustable LF biaspower and HF bias power supplies to the workpiece (block 212). Thecontroller 142 adjusts the ion energy level and ion energy distribution(width or spectrum) at the workpiece surface by simultaneous adjustmentsof the two RF bias power generators 132, 134 (block 214). This step iscarried out by any one of the following: One way is to adjust the ratiobetween the power levels of the HF and LF bias power sources 132, 134(block 214 a of FIG. 2). Another (less practical) way is adjusting orselecting the frequencies of the LF and HF bias power sources (block 214b of FIG. 2). In a first embodiment, the LF and HF frequencies areapplied to the ESC electrode 130 while the VHF source power is appliedto the gas distribution showerhead 110 (in which case the showerhead 110is the CCP applicator 116) while the ICP applicator 114 v overlies theshowerhead 110. In a second embodiment, the VHF source power is appliedto the ESC electrode 130 along with the HF and LF bias frequencies,while the ICP power applicator 114 overlies the showerhead 110.

If the method is used in an etch process for etching successive layersof different materials of a multilayer structure, the plasma processesfor etching each of the layers may be customized to be completelydifferent processes. One layer may be etched using highly dissociatedion and radical species while another layer may be etched in a higherdensity plasma than other layers, for example. Furthermore, if chamberpressure is changed between steps, the effects of such a change uponradial ion density distribution may be compensated in order to maintaina uniform distribution. All this is accomplished by repeating theforegoing adjustment steps upon uncovering successive layers of themultilayer structure (block 216).

The superior uniformity of plasma ion radial distribution achieved inthe step of block 210 makes it unnecessary to provide a large chambervolume above the wafer. Therefore, the distance between the wafer andthe plasma source may be reduced without compromising uniformity. Thismay be done when the reactor is constructed, or (preferably) the wafersupport 103 may be capable of being lifted or lowered relative to theceiling 108 to change the ceiling-to-wafer distance. By thus decreasingthe chamber volume, the process gas residency time is decreased,providing independent control over dissociation and plasma speciescontent. Also, reducing the ceiling-to-wafer distance permits the gasdistribution effects of the gas distribution showerhead 109 to reach thewafer surface before being masked by diffusion, a significant advantage.Thus, another step of the method consists of limiting theceiling-to-wafer distance to either (a) limit residency time or (b)prevent the showerhead gas distribution pattern from being masked at thewafer surface by diffusion effects (block 218 of FIG. 2). One advantageis that inductive coupling can now be employed without requiring a largeceiling-to-wafer distance to compensate for the center-low iondistribution characteristic of an inductively coupled source. In fact,the ceiling-to-wafer distance can be sufficiently small to enable anoverhead gas distribution showerhead to affect or improve processuniformity at the wafer surface.

The chemical species content of the plasma may be adjusted or regulatedindependently of the foregoing adjustments (e.g., independently of theadjustment of the radial ion density distribution of the step of block210) by adjusting the degree of dissociation in the plasma, in the stepof block 220 of FIG. 2. This step may be carried out by adjusting therate at which the chamber 104 is evacuated by the vacuum pump 160 (block220 a of FIG. 2), for example by controlling the valve 162, in order tochange the process gas residency time in the chamber. (Dissociationincreases with increasing residency time and increasing chamber volume.)Alternatively (or additionally), the adjustment of dissociation may becarried out by adjusting the ceiling-to-wafer distance so as to alterthe process gas residency time in the chamber (block 220 b of FIG. 2).This may be accomplished by raising or lowering the workpiece support103 of FIG. 1. The foregoing measures for adjusting dissociation in theplasma do not significantly affect the ratio of inductive and capacitivecoupling that was established in the step of block 210 for adjusting iondistribution or uniformity. Thus, the adjustment of the dissociation orchemical species content of step 220 is made substantially independentlyof the adjustment of plasma ion density distribution of step 210.

In an alternative embodiment, the capacitively coupled source powerapplicator 116 consists of electrodes in both the ceiling 108 and theworkpiece support 103, and VHF power is applied simultaneously throughthe electrodes in both the ceiling 108 and the workpiece support 103.The advantage of this feature is that the phase of the VHF voltage (orcurrent) at the ceiling may be different from the phase at the workpiecesupport, and changing this phase difference changes the radialdistribution of plasma ion density in the chamber 104. Therefore, anadditional step for adjusting the radial distribution of plasma iondensity is to adjust the phase difference between the VHF voltage (orcurrent) at the workpiece support 103 and the VHF voltage (or current)at the ceiling 108. This is indicated in block 230 of FIG. 2. Thisadjustment may or may not require changing the ratio between capacitiveand inductive coupling selected in the step of block 210.

FIGS. 3A, 3B and 3C show how the combination of a center-low or“M”-shaped inductively coupled plasma ion density distribution (FIG. 3A)with a center-high capacitively coupled plasma ion density distribution(FIG. 3B) results in a more ideal or more nearly uniform plasma iondensity distribution (FIG. 3C) that corresponds to the superposition ofthe distributions of FIGS. 3A and 3B. The ideal distribution of FIG. 3Cis achieved by a careful adjustment of the amount of inductive andcapacitive coupling of the two sources 118, 122 of FIG. 1. A high ratioof capacitively coupled power leads to a more center-high distribution,while a high ratio of inductively coupled power leads to a morecenter-low distribution. Different ratios will result in the idealdistribution at different chamber pressures. One way of apportioninginductive and capacitive coupling is to apportion the amount of RF powerof the two generators 118, 122. FIG. 4 depicts how the ratio between theoutput power levels of the generators 118, 122 affects the radial iondistribution. The minimum or dip in the curve of FIG. 4 corresponds toan ideal power ratio at which the non-uniformity or deviation in iondistribution is the least. Another way of apportioning betweeninductively and capacitively coupled power is to pulse at least one (orboth) of the two generators 118, 122, and control the pulse duty cycle.For example, one of them (the inductive source 118) may be pulsed andthe other (the capacitive source 122) may be continuous, and the two arebalanced by adjusting the duty cycle of the capacitively couple source122. Alternatively, both may be pulsed, and apportioning is done bycontrolling the ratio of the duty cycles of the two sources. The resultsare depicted in FIG. 5, in which a high ratio of inductivelycoupled-to-capacitively coupled duty cycles results in more inductivelycoupled power reaching the plasma and a more center-low distribution, Ahigh ratio of capacitively coupled power-to-inductively coupled powerresults in more capacitively coupled power in the plasma, providing acenter-high distribution.

The foregoing adjustments to the ion density distribution can be carriedout without changing plasma ion density. FIG. 6 illustrates how this isaccomplished in the embodiment of FIG. 4 in which uniformity adjustmentsare made by adjusting RF generator output power. FIG. 6 depicts lines ofconstant ion density for different combinations of inductively coupledpower (vertical axis) and capacitively coupled power (horizontal axis).Provided that the values of inductively and capacitively coupled powerfrom the generators 118, 122 respectively are constrained to lie along aparticular one of the lines of constant density, theinductive-capacitive power ratio may be set to any desired value (inorder to control uniformity) without changing the plasma ion density.The lines of constant density are deduced for any given reactor byconventional testing. FIG. 7 illustrates how this is accomplished in theembodiment of FIG. 5 in which uniformity adjustments are made byadjusting RF generator pulsed duty cycle. FIG. 7 depicts lines ofconstant ion density for different combinations of inductively coupledduty cycle (vertical axis) and capacitively coupled duty cycle(horizontal axis). Provided that the values of inductively andcapacitively coupled duty cycles from the generators 118, 122respectively are constrained to lie along a particular one of the linesof constant density, the inductive-capacitive power ratio may be set toany desired value (in order to control uniformity) without changing theplasma ion density. The lines of constant density are deduced for anygiven reactor by conventional testing.

FIG. 8 is a graph depicting the effect of the selection of the frequencyof the VHF capacitively coupled power source 122 upon ion density, inthe step of block 210 c of FIG. 2. FIG. 8 shows that ion density (andhence power coupling) increases with applied source power at a greaterrate as the frequency is increased (e.g., from 27 MHz, to 60 MHz andthen to 200 MHz). Thus, one way of affecting plasma ion density and thebalance between capacitive and inductively coupled power is to select orcontrol the VHF frequency of the capacitively coupled source RFgenerator 122.

FIG. 9 depicts a modification of the method of FIG. 2 in which a desiredplasma ion density is maintained while the inductive-to-capacitivecoupling ratio discussed above is employed to achieve a desired level ofdissociation or chemical species content of the plasma. The method ofFIG. 9 includes introducing process gas, preferably through the ceilinggas distribution showerhead 109 (block 302 of FIG. 9). The methodcontinues by capacitively coupling RF source power to the bulk plasma(block 304) while inductively coupling RF source power to the bulkplasma (block 306). The user establishes a certain plasma ion density inaccordance with a particular process step. This is accomplished bymaintaining the combined total of the capacitively coupled power and theinductively coupled power at a level providing the desired plasma iondensity for the process step to be carried out (block 308). At the sametime, the degree of dissociation in the bulk plasma is determined (e.g.,to satisfy a certain process requirement) while maintaining the desiredplasma ion density. This is accomplished by adjusting the ratio betweenthe amounts of the VHF capacitively coupled power and the inductivelycoupled power (block 310). This fixes the dissociation (kinetic electronenergy in the bulk plasma) between a very high level characteristic ofan inductively coupled plasma and a lower level characteristic of a VHFcapacitively coupled plasma. Such apportionment can be accomplishedwithout perturbing the ion density by maintaining the total RF powernearly constant while changing only the ratio between the powerdelivered by the HF and VHF generators 118, 122, in accordance with themethods described above with reference to FIG. 6 and (or) FIG. 7.

The adjustment of step 310 can be carried out by any one (or acombination) of the following step: A first type of adjustment consistsof adjusting the RF generator power levels of the inductively andcapacitively coupled power sources 118, 122 (block 310 a of FIG. 9).Another type consists of pulsing at least one or both of the inductivelyand capacitively coupled RF power generators 118, 122 and adjusting theduty cycle of one relative to the other (block 310 b of FIG. 9). A thirdtype consists of adjusting the effective frequency of the capacitivelycoupled power VHF generator 122 (block 310 c of FIG. 9), in which plasmaion density increases as the VHF frequency is increased. Changing theeffective VHF frequency can be carried out by providing a pair of fixedfrequency VHF generators 122 a, 122 b having respective frequencies andadjusting the ratio between their output power levels.

The method further includes coupling independently adjustable LF biaspower and HF bias power supplies to the workpiece (block 312). Thecontroller 142 adjusts the ion energy level and ion energy distribution(width or spectrum) at the workpiece surface by simultaneous adjustmentsof the two RF bias power generators 132, 134 (block 314). This step iscarried out by any one of the following: One way is to adjust the ratiobetween the power levels of the HF and LF bias power sources 132, 134(block 314 a of FIG. 9). Another way is to adjusting or selecting thefrequencies of the LF and HF bias power sources (block 314 b of FIG. 9).

The method is useful for performing plasma enhanced etch processes,plasma enhanced chemical vapor deposition (PECVD) processes, physicalvapor deposition processes and mask processes. If the method is used inan etch process for etching successive layers of different materials ofa multilayer structure, the plasma processes for etching each of thelayers may be customized to be completely different processes. One layermay be etched using highly dissociated ion and radical species whileanother layer may be etched in a higher density plasma than otherlayers, for example. Furthermore, if chamber pressure is changed betweensteps, the effects of such a change upon radial ion density distributionmay be compensated in order to maintain a uniform distribution. All thisis accomplished by repeating the foregoing adjustment steps uponuncovering successive layers of the multilayer structure (block 316).

The superior uniformity of plasma ion radial distribution achieved bycombining inductively coupled source power and VHF capacitively coupledsource power makes it unnecessary to provide a large ceiling-to-waferdistance. Therefore, the ceiling-to-wafer distance may be reducedwithout compromising uniformity. This may be done when the reactor isconstructed, or (preferably) the wafer support 103 may be capable ofbeing lifted or lowered relative to the ceiling 108 to change theceiling-to-wafer distance. By thus decreasing the chamber volume, theprocess gas residency time is decreased, providing independent controlover dissociation and plasma species content. Also, reducing theceiling-to-wafer distance permits the gas distribution effects of thegas distribution showerhead 109 to reach the wafer surface before beingmasked by diffusion, a significant advantage. Thus, another step of themethod consists of limiting the ceiling-to-wafer distance to either (a)limit residency time or (b) prevent the showerhead gas distributionpattern from being masked at the wafer surface by diffusion effects(block 318 of FIG. 9).

The chemical species content of the plasma may be adjusted or regulatedindependently of the foregoing adjustments by adjusting the process gasresidency time in the chamber, in the step of block 320 of FIG. 9. Thisstep may be carried out by adjusting the rate at which the chamber 104is evacuated by the vacuum pump 160 (block 320 a of FIG. 9), for exampleby controlling the valve 162, in order to change the process gasresidency time in the chamber. (Dissociation increases with increasingresidency time.) Alternatively (or additionally), the adjustment ofdissociation may be carried out by adjusting the ceiling-to-waferdistance so as to alter the process gas residency time in the chamber(block 320 b of FIG. 9). This may be accomplished by raising or loweringthe workpiece support 102 of FIG. 1. The foregoing measures foradjusting dissociation in the plasma do not significantly affect theratio of inductive and capacitive coupling that was established in thestep of block 310. Thus, the adjustment of the dissociation or chemicalspecies content of step 320 is made substantially independently of (orin addition to) the adjustment of dissociation of step 210.

In an alternative embodiment, the capacitively coupled source powerapplicator 116 consists of electrodes in both the ceiling 108 and theworkpiece support 103, and VHF power is applied simultaneously throughthe electrodes in both the ceiling 108 and the workpiece support 103.The advantage of this feature is that the phase of the VHF voltage (orcurrent) at the ceiling may be different from the phase at the workpiecesupport, and changing this phase different changes the radialdistribution of plasma ion density in the chamber 104. Therefore, theradial distribution of plasma ion density may be adjusted independentlyof the dissociation (i.e., without changing the capacitive-to-inductivecoupling ratio selected in the step of block 310) by adjusting the phasedifference between the VHF voltage (or current) at the workpiece support103 and the VHF voltage (or current) at the ceiling 108. This isindicated in block 330 of FIG. 9.

FIG. 10 is a graph depicting how the ratioing of inductive andcapacitive coupling controls dissociation in the bulk plasma in the stepof block 308. Dissociation is promoted by an increase in electron energywithin the bulk plasma, and FIG. 10 depicts the electron energydistribution function for four different operating regimes.

The curve labeled 410 depicts the electron energy distribution functionin the case in which only the HF bias power is applied to the wafer andno source power is applied. In this case, the electron population isconfined within a low energy spectrum, well below an energy at which thecross-section for a typical dissociation reaction (represented by thecurve 420) has an appreciable magnitude. Therefore, less (if any)dissociation occurs.

The curve labeled 430 depicts the electron energy distribution functionin the case in which VHF power is applied to the capacitively coupledsource power applicator 116 and no power is applied to any otherapplicator. In this case, the electron population has a small componentcoinciding with the collision cross-section 420 and so a small amount ofdissociation occurs.

The curve labeled 440 depicts the electron energy distribution functionin the case in which HF power is applied to the inductively coupledsource power applicator 114 and power is applied to no other applicator.In this case, the electron population has a component coinciding with ahigh value of the collision cross-section 420, and therefore a very highdegree of dissociation occurs in the bulk plasma.

The curve labeled 450 depicts the electron energy distribution functionfor a case in which RF power is apportioned between the capacitive andinductively coupled applicators 116, 114. In this case, the resultingelectron energy distribution function is mixture of the two functions430, 440 and lies between them, so that a lesser amount of iondissociation occurs in the bulk plasma. The curve 450 representing thecombined case has a somewhat smaller electron population at or above anenergy at which the collision cross-section 420 has a significantmagnitude, leading to the lesser degree of dissociation. The combinationcase curve 450 can be shifted toward greater or lesser energy levels bychanging the ratio between the amounts of capacitive and inductivecoupled power. This is depicted in the graph of FIG. 11 in which eachsolid line curve corresponds to the electron energy distributionfunction for purely inductively coupled power at a particular powerlevel. The dashed line curves extending from the solid line curvesdepict the modification of those curves as more power is diverted awayfrom inductive coupling and applied to capacitive coupling. Essentially,this causes the electron population to shift to lower energy levels,thereby decreasing dissociation.

FIG. 12 illustrates the effects of different levels of dissociation uponthe chemical content of the plasma. The vertical axis represents theoptical emission spectrum intensity and the horizontal axis representswavelength. Different peaks correspond to the presence of certainradicals or ions, and the magnitude of the peak corresponds to thepopulation or incidence in the plasma of the particular species. Thesolid line curve corresponds to a low degree of dissociation (capacitivecoupling predominant), in which larger molecular species are present inlarge numbers. The dashed line curve corresponds to a high degree ofdissociation (inductive coupling predominant), in which smaller (morereactive) chemical species are present in large numbers (depending uponthe parent molecule). In the example illustrated in FIG. 12, a largemolecular-weight species with high incidence in the predominantlycapcitively coupled regime is CF2, while a low molecular-weight specieswith high incidence in the predominantly inductively coupled regime isfree carbon C. In some cases, the presence of C (free carbon) is anindicator of the presence of very light and highly reactive species,such as free fluorine, which may be desirable where a high etch rate isdesired. The presence of the larger species such as CF2 is an indicatorof less dissociation and an absence of the more reactive species, whichmay be desirable in a plasma etch process requiring high etchselectivity, for example.

FIG. 13 is a graph illustrating one way of carrying out the step ofblock 310 a of FIG. 9. The vertical axis of FIG. 13 corresponds to thedegree of dissociation in the bulk plasma, and may represent the opticalemission spectrum intensity of a highly dissociated species such as freecarbon in FIG. 12. The horizontal axis is the ratio of inductivelycoupled plasma (ICP) power to capacitively coupled plasma (CCP) power(the power levels of the ICP and CCP generators 118, 122 of FIG. 1).FIG. 13 indicates that the dissociation is a generally increasingfunction of this ratio, although it may not be the simple linearfunction depicted in FIG. 13.

FIG. 14 is a graph illustrating one way of carrying out the step ofblock 310 b of FIG. 9. The vertical axis of FIG. 14 corresponds to thedegree of dissociation in the bulk plasma, and may represent the opticalemission spectrum intensity of a highly dissociated species such as freecarbon in FIG. 12. The horizontal axis is the ratio of inductivelycoupled plasma (ICP) pulsed duty cycle to capacitively coupled plasma(CCP) pulsed duty cycle (the pulsed duty cycles of the ICP and CCPgenerators 118, 122 of FIG. 1). FIG. 14 indicates that the dissociationis a generally increasing function of this ratio, although it may not bethe simple linear function depicted in FIG. 14. The CCP generator 122may not be pulsed, in which case its duty cycle is 100%, while only theICP duty cycle is varied to exert control. FIGS. 15A and 15B illustrateone possible example of the contemporaneous waveforms of the pulsed ICPgenerator output and the pulsed CCP generator output. In thisillustrated example, the CCP generator 122 has a higher duty cycle thanthe ICP generator 118, so that the plasma is likely to exhibit more thecharacteristics of a capacitively coupled plasma, such as a low degreedissociation. The ratio between the duty cycles of the capacitively andinductively coupled power sources affects the proportion betweeninductively and capacitively coupled power in the plasma in thefollowing way. First, the shorter the duty cycle of the inductivelycoupled power source, the longer the idle time between the pulsed burstsof RF inductive power. During the idle time, the highest energyelectrons in the bulk plasma loose their energy faster than other lessenergetic electrons, so that the electron energy distribution function(FIG. 10) shifts downward in energy (i.e., to the left in FIG. 10). Thisleads to a more capacitively coupled-like plasma (i.e., lessdissociation) during each idle time. This effect increases as duty cycleis decreased, so that the plasma has (on average over many cycles) lesshigh energy electrons, leading to less dissociation. During the idletime, the higher energy electron distribution decays, and (in addition)spatial distribution of the higher energy electrons has an opportunityto spread through diffusion, thus improving process uniformity to adegree depending upon the reduction in inductively coupled power dutycycle.

FIG. 16 is a graph depicting one way of carrying out the step of block310 c of FIG. 9. The vertical axis of FIG. 16 corresponds to the degreeof dissociation in the bulk plasma, and may represent the opticalemission spectrum intensity of a highly dissociated species such as freecarbon in FIG. 12. The horizontal axis is the frequency of thecapacitively coupled plasma (CCP) generator 122 of FIG. 1. FIG. 16corresponds to the case in which both CCP and ICP power is appliedsimultaneously, as in the previous examples, and the frequency of theCCP power generator 122 is increased. For a fixed level of ICP power anda fixed level of CCP power, increasing the effective VHF frequencyincreases the plasma dissociation, as indicated in FIG. 16. Thedissociation behavior may not be the simple linear function depicted inFIG. 16.

FIGS. 17A, 17B and 17C illustrate how the step of block 214 of FIG. 2(which corresponds to or is the same as the step of block 314 of FIG. 9)is carried out. Each of the graphs of FIGS. 17A, 17B, 17C depicts thepopulation of ions at the plasma sheath (at the workpiece surface) as afunction of ion energy, or the sheath ion energy distribution.

FIG. 17A depicts the ion energy distribution in the case in which theonly bias power that is applied to the wafer is a low frequency (e.g., 1MHz) bias voltage or current. (In FIG. 1, this corresponds to the casein which only the LF bias power generator 132 applies bias power.) Thisfrequency is substantially below the sheath ion transit frequency, whichis the highest frequency at which the sheath ions can follow anoscillation of the sheath electric field. Therefore, the sheath ions inthe example of FIG. 17A can follow the peak-to-peak oscillations of thesheath electric field imposed by the bias power. This results in a peakion energy that coincides with the RF bias power peak-to-peak voltage(labeled eVp-p in FIG. 17A). The ion energy distribution is bi-modal andhas a second peak at a much lower energy, as depicted in the graph ofFIG. 17A. The ion distribution between these two peaks is relativelylow.

FIG. 17B depicts the ion energy distribution in the case in which thebias power consists only of a high frequency (HF) component (such as13.56 MHz). (In FIG. 1, this corresponds to the case in which only theHF bias power generator 134 applies bias power.) This frequency is wellabove the sheath ion transit frequency, and therefore the sheath ionsare unable to follow the peak-to-peak sheath electric field oscillation.The result is that the ion energy distribution of FIG. 17B is confinedto a narrow energy band centered at half of the peak-to-peak voltage ofthe sheath. The ion energy distributions of FIGS. 17A and 17B can beseen to be somewhat complementary to one another, with one distribution(FIG. 17B) being rich in a middle frequency band while the other (FIG.17A) peaks at two extremes, has a wide distribution that is somewhatdepleted at the middle frequencies.

FIG. 17C illustrates an example of an ion energy distribution that canbe realized by applying both LF and HF bias power simultaneously (byenabling both bias power generators 132, 134 of FIG. 1). This results inan ion energy distribution that is, in effect, a superposition of thetwo extreme distributions of FIGS. 17A and 17B. The “combination” ionenergy distribution of FIG. 17C is therefore adjustable by adjusting therelative amounts of LF and HF bias power. This is accomplished by either(or both) apportioning the power levels of the LF and HF bias powergenerators 132, 134 (as in step 214 a of FIG. 2) or pulsing one or bothof them and apportioning their duty cycles (as in step 214 b of FIG. 2).Alternatively, or as an additional step, the frequency of either the HFor the LF bias power may be changed. For example, the LF bias powerfrequency may be increased to a value closer to the sheath ion transitfrequency, which would reduce the ion energy distribution populationnear the maximum energy (eVp-p) in FIG. 17C (thereby narrowing the ionenergy distribution as indicated by the dotted line curve of FIG. 17C).As another example, the HF bias power frequency can be reduced to avalue closer to the sheath ion transit frequency, which would decreasethe distribution peak at the intermediate energies of FIG. 17C (therebybroadening the ion energy distribution in the middle frequencies asindicated by the dashed line of FIG. 17C).

FIG. 18 depicts a multilayer thin film structure of a typical gate of atypical field effect transistor (FET). These layers include a highdielectric constant silicon dioxide layer 602 overlying a semiconductorsubstrate 604, a polycrystalline silicon conductive layer 606 on theoxide layer 602, a titanium silicide layer 608 on the conductive layer606, a hard mask layer 610 over the silicide layer 608, ananti-reflective (AR) coating 612 on the hard mask layer 610 and aphotoresist layer 614 on the AR coating 612. In a plasma etch processfor etching such a structure, the different materials of each of thelayers 602-614 is best etched in a different etch process. Some of thelayers (e.g., the photoresist layer 614 and the polycrystalline siliconconductive layer 606 are best etched in a plasma that is moreinductively coupled than capacitively coupled, while other layers (e.g.,the hard mask layer 610) are best etched in plasma that is morecapacitively coupled than inductively coupled. Using the methods of FIG.2 or FIG. 9, each of the different layers may be processed (e.g.,etched) with the type of plasma process conditions that are optimal forthat particular layer, by changing the process conditions, including thetype of source power coupling (i.e., changing the ratio betweeninductively and capacitively coupled source power). Thus, in an etchprocess, as each successive layer 602-614 is exposed, the adjustmentsdescribed with reference to FIGS. 1 and 9 are repeated to change theprocess parameters to customize the process for each layer. This is thegoal of the step of blocks 216 and 316 of FIGS. 2 and 9 respectively. Inmaking such changes, other process parameters may be changed. Forexample, a predominantly inductively coupled plasma of the type used toetch the polycrystalline layer 606 may be better maintained at a lowerchamber pressure (e.g., a several milliTorr), while a predominantlycapacitively coupled plasma may be better maintained at a higher chamberpressure (e.g., tens of milliTorr). Plasmas having nearly the sameamount of inductively and capacitively coupled power may be operated atchamber pressures intermediate the higher chamber pressure range of acapacitively coupled plasma and the lower pressure range of aninductively coupled plasma. Moreover, different bias power levels andion energy distributions may be employed to etch different ones of thelayers 602-614, using the steps of blocks 214 or 314 of FIGS. 1 or 9 tomake the adjustments.

Advantages:

The simultaneous application of both VHF capacitively coupled power andinductively coupled power to the plasma enables the user toindependently control plasma ion density and either plasma uniformity ordissociation (or chemical species content of the plasma). Conventionalreactors compensate for the center-low ion density distribution of aninductively coupled plasma by applying power from the ceiling using ahigh ceiling-to-wafer distance so that diffusion effects produce auniform plasma ion distribution at the wafer. However, such a largeceiling-to-wafer distance would mask the desired effects of an overheadgas distribution showerhead at the wafer surface, so that the benefitsof an overhead gas distribution showerhead could not be realized in aninductively coupled reactor. Another problem is that the largeceiling-to-wafer spacing renders the chamber volume very large, so thatthe process gas residency time is correspondingly large (unless anextremely high capacity vacuum pump evacuates the chamber), making itdifficult to control dissociation in the bulk plasma below a minimumlevel. This has made it more difficult to minimize or solve etchprocessing problems such as etch microloading or lack of etchselectivity. These problems are all solved in the invention. The seeminginability to employ an overhead gas showerhead in an inductively coupledreactor to improve process uniformity at the wafer surface is solved byintroducing an ideal amount of capacitively coupled power to make theion distribution uniform in the ion generation region. This permits theceiling-to-wafer spacing to be greatly reduced to the point that anoverhead gas showerhead controls process uniformity at the wafersurface. Etch selectivity is improved and etch microloading is reducedby reducing dissociation in the plasma through the reduced gas residencytime of the smaller chamber volume facilitated by the reducedceiling-to-wafer distance. In addition, the etch microloading problemmay be solved by independent means by selecting a desired chemicalcontent of the plasma by promoting the degree of dissociation thatpromotes the desired chemical species. Certain chemical species cansuppress the effects of etch microloading, and by adjusting the ratio ofthe capacitively coupled power to inductively coupled power, thedissociation may be varied to maximize the amount of the desired speciespresent in the plasma. Another advantage is that all of this can beperformed while maintaining the overall plasma ion density at a desiredlevel, or independently adjusting plasma ion density.

Apparatus:

FIG. 19 illustrates a first embodiment of a plasma reactor of theinvention for processing a workpiece 102, which may be a semiconductorwafer, held on a workpiece support 103 within a reactor chamber 104.Optionally, the workpiece support 103 be raised and lowered by a liftservo 105. The chamber 104 is bounded by a chamber sidewall 106 and aceiling 108. The ceiling 108 may include a gas distribution showerhead109 having small gas injection orifices 110 in its interior surface, theshowerhead 109 receiving process gas from a process gas supply 112. Thereactor includes an inductively coupled RF plasma source powerapplicator 114. As illustrated in FIG. 22, the inductively coupled powerapplicator may consist of a conductive coil 114 a wound in a helix andlying over the ceiling 108 in a plane parallel to the ceiling 108.Alternatively, as depicted in FIG. 23, the conductive coil may consistof parallel helically wound conductors 114 b, 114 c, 114 d. Acapacitively coupled RF plasma source power applicator 116, in oneembodiment, is an electrode 116 a in the ceiling overlying the gasdistribution showerhead. In another embodiment, the capacitively coupledplasma source power applicator 116 is an electrode 130 within theworkpiece support 130. In order to permit inductive coupling into thechamber 104 from the coil antenna 114 a, the gas distribution showerhead109 may be formed of a dielectric material such as a ceramic. Theceiling electrode 116 a preferably has multiple radial slots 115 asillustrated in FIG. 20 to permit inductive coupling into the chamber 104from the overhead coil antenna 114 a into the chamber. Alternatively, aceiling electrode 116 b depicted in FIG. 21 may be employed that is notslotted and instead is formed of a material capable of functioning as anelectrode while at the same time permitting inductive coupling of RFpower from the overhead coil antenna 114. One example of such a materialis a doped semiconductor.

In an alternative embodiment, the capacitively coupled source powerapplicator 116 may include both the electrode 116 a within the ceiling108 and the electrode 130 within the workpiece support 103, so that RFsource power may be capacitively coupled simultaneously from the ceiling108 and the workpiece support 103. In yet another alternativeembodiment, both electrodes 116 a and 130 are present, but VHF sourcepower is applied to only one of them while the other serves as an VHFreturn or counter electrode.

An RF power generator 118 provides high frequency (HF) power (e.g.,within a range of about 10 MHz through 27 MHz) through an impedancematch element 120 to the inductively coupled coil antenna 114 a. In oneembodiment in which the ceiling electrode 116 a is the capacitivelycoupled source power applicator, an RF power generator 122 provides veryhigh frequency (VHF) power (e.g., within a range of about 27 MHz through200 MHz) through an impedance match element 124 to the capacitivelycoupled power applicator 116. In another embodiment in which the bottom(workpiece support) electrode 130 is the capacitively coupled sourcepower applicator, an RF power generator 123 provides VHF power throughan impedance match element 125 to the bottom electrode 130. In a thirdembodiment, both the ceiling and bottom electrodes 116 a, 130 comprisethe capacitively coupled plasma source power applicator, so that bothVHF generators 122, 123 are present. In a further embodiment, bothelectrodes 116 a, 130 are present, but VHF plasma source power isapplied to only one them, while the other is coupled to the VHF returnpotential (e.g., ground) in order to serve as a counterelectrode for theother.

The efficiency of the capacitively coupled power source applicator 116in generating plasma ions increases as the VHF frequency increases, andthe frequency range preferably lies in the VHF region for appreciablecapacitive coupling to occur. Power from both RF power applicators 114,116 is coupled to a bulk plasma 126 within the chamber 104 formed overthe workpiece support 103.

RF plasma bias power is coupled to the workpiece 102 from an RF biaspower supply coupled to the electrode 130 inside the workpiece supportand underlying the wafer 102. The RF bias power supply may include a lowfrequency (LF) RF power generator 132 (100 kHz to 4 MHz) and another RFpower generator 134 that may be a high frequency (HF) RF power generator(4 MHz to 27 MHz). An impedance match element 136 is coupled between thebias power generators 132, 134 and the workpiece support electrode 130.A vacuum pump 160 evacuates process gas from the chamber 104 through avalve 162 which can be used to regulate the evacuation rate. Theevacuation rate through the valve 162 and the incoming gas flow ratethrough the gas distribution showerhead 109 determine the chamberpressure and the process gas residency time in the chamber. If theworkpiece support 103 is an electrostatic chuck, then a D.C. chuckingvoltage supply 170 is connected to the electrode 130. A capacitor 172isolates the RF generators 123, 132, 134 from the D.C. voltage supply170.

In the first embodiment, VHF power is applied only to the ceilingelectrode 116 a. In this case, it may desirable for the workpiecesupport electrode 130 to function as the return path for the VHF powerapplied to the ceiling electrode 116 a and for the ceiling electrode tofunction as the return path for the HF power applied to the workpiecesupport electrode 130. For this purpose, the ceiling electrode 116 a maybe connected through an LF/HF bandpass filter 180 to ground. Thebandpass filter 180 prevents VHF from the generator 122 from beingdiverted from the ceiling electrode 116 a to ground. Similarly, thewafer support electrode 130 may be connected (via the RF isolationcapacitor 172) to ground through a VHF bandpass filter 186. The VHFbandpass filter 186 prevents LF and HF power from the generators 132,134 from being diverted from the electrode 130 to ground.

In the second embodiment, VHF power is applied to only the wafer supportelectrode 130. In this case, the wafer support electrode 130 is notconnected to ground, but rather to the VHF generator 123 (via the match125), so that the VHF bandpass filter 186 is eliminated. Likewise, theLF/HF bandpass filter 180 may be bypassed (or eliminated) and theceiling electrode 116 a connected directly to ground. The foregoingoptions are indicated symbolically by the switches 184, 188 in FIG. 19.It is understood that the reactor may be permanently configured inaccordance with one of the first or second embodiments rather than beingconfigurable (by the switches 184, 188) into either embodiment, so thatonly one of the VHF generators 122, 123 would be present, and theswitches 184, 188 would be unnecessary in such a case.

In the third embodiment, both electrodes 116 a, 130 are drivensimultaneously by the VHF generators 122, 123 so that neither could be aVHF ground. However, the ceiling electrode 116 a could be connectedthrough the LF/HF bandpass filter 180 to ground in order to be acounterelectrode or return for LF/HF bias power applied to the wafersupport electrode 130. In this embodiment, the side wall 106 may providea ground return for the VHF power. If the VHF phase between the twoelectrodes 130, 116 a is different, then each electrode may provide somereference potential for at least a portion of each RF cycle. Forexample, the VHF phase difference between the two electrodes 116 a, 130were 180 degrees, then each electrode 116 a, 130 would function as acounterelectrode for the other during the entirety of each RF cycle. Thetwo VHF generators 122, 123 may be realized in a single VHF generator,with a source power controller 140 governing the difference in phasebetween the VHF voltages or the VHF currents delivered by the singlegenerator to the respective electrodes 116 b, 130.

The source power controller 140 regulates the source power generators118, 122 independently of one another in order to control bulk plasmaion density, radial distribution of plasma ion density and dissociationof radicals and ions in the plasma. The controller 140 is capable ofindependently controlling the output power level of each RF generator118, 122. In addition, or alternatively, the controller 140 is capableof pulsing the RF output of either one or both of the RF generators 118,122 and of independently controlling the duty cycle of each, or ofcontrolling the frequency of the VHF generator 122 and, optionally, ofthe HF generator 118. The controller 140 may also control the pumpingrate of the vacuum pump 160 and/or the opening size of the evacuationvalve 162. In addition, a bias power controller 142 controls the outputpower level of each of the bias power generators 132, 134 independently.The controllers 140, 142 are operated to carry out the various methodsof the invention described above.

FIG. 24 illustrates another modification of the embodiment of FIG. 19 inwhich the coil antenna 114 a includes one (or more) solenoidal conductorwindings 190, 192 fed by respective RF generators 194 a, 194 a throughrespective impedance matches 196 a, 196 b. In this case, the ceiling 108and showerhead 109 may be either flat (solid line) or dome shaped(dotted line). FIG. 25 depicts a modification of the embodiment of FIG.19 in which the ceiling 108 and gas distribution showerhead 109 have acenter-high stepped shaped. In this case the coil antenna 114 a canassume either a flat shape (dotted line) or a hemispherical (or dome)shape as shown in solid line in FIG. 25. FIG. 26 depicts anothermodification of the embodiment of FIG. 19 in which the ceiling 108 andthe gas distribution showerhead 109 are hemispherical or dome shaped.Again, the coil antenna 114 a be flat (dotted line) or dome shaped(solid line).

FIG. 27 illustrates another embodiment in which the inductively coupledsource power applicator 114 is a toroidal source rather than aninductive antenna. The toroidal source consists of an external hollowreentrant conduit 402 coupled to a pair of openings 404, 406 in thechamber enclosure that are separated by the diameter of the processregion. For example, in the implementation of FIG. 27, the openings 404,406 are through the ceiling 108 and are at the edge of the chamber sothat they are separated by the diameter of the wafer support 103. RFpower is coupled into the interior of the conduit 402 by means of amagnetic (e.g., iron) toroidal core 408 having a conductive winding 409wrapped around a portion of the core 408. The RF generator 118 iscoupled through the match 120 to the winding 409. This toroidal sourceforms a plasma current in a circular path that passes through theconduit 402 and through the processing region overlying the wafer 102.This plasma current oscillates at the frequency of the RF generator 118.FIG. 28 depicts a modification of the reactor of FIG. 27 in which theceiling 108 and showerhead 109 are a center high step shape (solid line)or dome shaped (dotted line). One advantage of the toroidal plasmasource of FIGS. 27 and 28 is that RF power is not inductive coupleddirectly through the gas distribution showerhead 109 nor through theceiling electrode 116 b. Therefore, the showerhead 109 may be metal andthe ceiling electrode 116 a may be solid (without the slots 115 of FIG.20), or the ceiling electrode may be eliminated and the VHF powercoupled directly to the metal gas distribution showerhead 109 so thatthe metal showerhead 109 functions as the ceiling electrode.

Each of the reactors of FIGS. 19-26 capacitively couples VHF sourcepower into the chamber while inductively coupling HF source power intothe chamber. The reactors of FIGS. 27-28 capacitively couple VHF sourcepower into the chamber and inductively couple HF source power to anoscillating toroidal plasma current that passes through the processregion of the chamber. This inductive coupling element faces an externalportion of the oscillating toroidal plasma current. The capacitivelycoupled power is applied in the embodiments of FIGS. 19-26 to theceiling electrode 116 a or to the wafer support electrode 116 b, and isapplied in the embodiments of FIGS. 27-28 to a conductive version of theshowerhead 109 (or to the wafer support electrode 116 b). Thecapacitively coupled power generates ions in the bulk plasma because itis in the VHF frequency range (27-200 MHz). In this frequency range,kinetic electrons in the bulk plasma follow the capacitively coupled RFfield oscillations and therefore acquire sufficient energy to contributeto ion generation. Below this range, the capacitively coupled powerwould contribute more to ion energy in the plasma sheath rather than toion generation in the bulk plasma, and therefore would not be plasmasource power. Therefore, in order to provide plasma source power (i.e.,power for generating ions in the bulk plasma), the RF generator 122 (or123) coupled to the electrode 116 a (or 130) provides VHF power.

While control over all process parameters has been described as beingcarried out by two controllers 140, 142, it is understood that thecontrollers may be realized in a single controller that controls allprocess parameters and adjustments.

The foregoing methods are applicable to plasma processing of asemiconductor wafer or plasma processing of a plasma display substrate.

1. A plasma reactor for processing a workpiece, comprising: a reactorchamber and a workpiece support within said chamber, said chamber havinga ceiling facing said workpiece support; a capacitively coupled plasmasource power applicator comprising a source power electrode at one of:(a) said ceiling (b) said workpiece support, and plural VHF powergenerators of different fixed frequencies coupled to said capacitivelycoupled source power applicator; a controller for independentlycontrolling the power output levels of said plural VHF generators so asto control an effective VHF frequency applied to said source powerelectrode; a plasma bias power applicator comprising a bias powerelectrode in said workpiece support and at least a first RF bias powergenerator coupled to said plasma bias power applicator; wherein saidplural VHF generators comprise a first VHF generator of fixed VHFfrequency f₁ with output power level a₁ and a second VHF generator offixed VHF frequency f₂ with output power level a₂ and wherein saidcontroller governs said output power levels to control an effective VHFfrequency f_(eff) within a range from f₁ to f₂ in accordance withf_(eff)=[a₁f₁+a₂f₂]/(a₁+a₂).
 2. The reactor of claim 1 wherein saidsource power electrode is at said ceiling and comprises: process gasdistribution apparatus comprising a gas distribution showerhead in saidceiling.
 3. The reactor of claim 2 wherein said VHF source powergenerator is coupled to said electrode at said ceiling, said reactorfurther comprising a VHF bandpass filter connected between said biaspower electrode and ground whereby said bias power electrode is acounterelectrode for said electrode at said ceiling.
 4. The reactor ofclaim 3 further comprising an LF/HF bandpass filter connected betweensaid ceiling electrode and ground, whereby said ceiling electrode is acounterelectrode for bias power applied to said bias power electrode. 5.The reactor of claim 2 wherein said ceiling and said gas distributionshowerhead have a dome shape.
 6. The reactor of claim 5 wherein saiddome shape is one of: (a) smooth, (b) stepped.
 7. The reactor of claim 6wherein said dome shape is center high relative a ceiling-to-workpiecedistance.
 8. The reactor of claim 1 further comprising: a second RF biaspower generator coupled to said bias power electrode, said first andsecond RF bias power generators providing RF power at a low frequencyand at a high frequency, respectively; a second controller capable ofadjusting the relative amounts of power simultaneously coupled to saidbias power electrode by said first and second RF bias power generators.9. The reactor of claim 8 wherein said source power electrode is at saidworkpiece support and wherein said source power electrode and said biaspower electrode are the same electrode.
 10. The reactor of claim 8wherein said low frequency and said high frequency are greater and lessthan, respectively, a sheath ion transit frequency of plasma in saidchamber.
 11. The reactor of claim 1 wherein said VHF source powergenerator is coupled to said bias power electrode at said workpiecesupport, said electrode at said ceiling being connected to groundwhereby said electrode at said ceiling is a counterelectrode for VHFpower applied to said bias power electrode and for HF or LF bias powerapplied to said bias power electrode.
 12. The reactor of claim 1 whereinsaid controller is capable of controlling the evacuation rate of saidchamber.